1. Technical Field
The present disclosure relates to devices and methods for use in orthopedic spine surgery. In particular, the present disclosure relates to a device and methods for coupling a spinal rod to a patient's vertebral body.
2. Background of Related Art
The human spine is comprised of thirty-three vertebrae at birth and twenty-four as a mature adult. The vertebra includes the vertebral body and posterior elements, including the spinous process, transverse processes, facet joints, laminae, and pedicles. The vertebral body consists of a cortical shell surrounding a cancellous center. Between each pair of vertebrae is an intervertebral disc, which maintains the space between adjacent vertebrae and acts as a cushion under compressive, bending and rotational loads and motions. A healthy intervertebral disc consists mostly of water in the nucleus pulposus, which is the center portion of the disc. The water content gives the nucleus a spongy quality and allows it to absorb spinal stresses.
There are many diseases, such as spondylolisthesis and scoliosis, and injuries that affect the vertebral and intervertebral discs. Excessive pressure or injuries to a disc can result in injury to the annulus, which is the outer ring that holds the disc together. Generally, the annulus is the first portion of the disc to be injured. These injuries are typically in the form of small tears that heal by forming scar tissue. The scar tissue is not as strong as normal annulus tissue and weakens over time. This can lead to damage of the nucleus pulposus. When damaged, the nucleus will begin to lose its water content and will dry up. As the nucleus loses its water content, it collapses, allowing the vertebrae above and below the disc space to move closer to one another. This results in a narrowing of the disc space between the vertebrae and an imbalance in the spinal column. As a result, the discs will lose their ability to act as shock absorbers or cushions. The increased load and wear due to the biomechanical imbalance of the spine initiates a cascading deleterious effect.
When a disc or vertebra is damaged due to disease or injury, standard practice is to remove part or all of the intervertebral disc, insert a natural or artificial disc spacer along with bone graft, and construct an internal bracing structure to hold the affected vertebrae in place in an attempt to achieve a spinal fusion. Typically, the structure is created by inserting screws through the pedicle and cortical shell of the vertebra and into the cancellous center of the vertebral body. Then, a rod is rigidly attached to the screws securing the vertebrae in place.
Osteoporosis, a disease that causes low bone mass and deterioration of bone tissue, is an additional challenge when trying to create an internal structure to stabilize the spine. When osteoporosis affects the spine, the lack of quality bone compromises the screw's ability to gain adequate purchase. Since osteoporosis affects the cancellous, more porous, bone before affecting the denser cortical bone, greater fixation is achievable by taking advantage of posterior elements that are mainly cortical bone rather than the vertebral body alone. For patients suffering from scoliosis, an abnormal curvature of the spine, posterior spinal elements also present a strong point of fixation from which manual correction of the spine may be applied and then internally braced.